Why does a sagging guy wire not behave like a rigid brace?

The short answer

A guy wire carries tension and changes shape as its endpoints move. Sag, initial tension and cable extension influence how much additional force develops. It therefore does not behave like a rigid brace with unlimited compression resistance. The tower assessment must represent the restraint supplied by the real cable geometry and tension state.

Distinguish axial extension from geometric movement

Cable material stiffness describes force associated with extension along its axis. Movement transverse to a cable also changes its direction and shape, creating a different restoring response. Initial tension is important to that response even before substantial material extension occurs.

A long cable with noticeable sag can accommodate endpoint movement partly through geometric change. Replacing that behavior with an infinitely stiff support can overstate the tower’s lateral restraint.

Check the range in which the model applies

As wind displaces the tower, one guy can gain tension while an opposing guy unloads. If a cable becomes slack, its restraint changes markedly. A constant linear spring may be useful near one equilibrium but not represent every extreme displacement.

The assessment should use actual attachment positions, anchor elevations, cable properties and pretension. Cable diameter alone cannot establish effective support stiffness or ensure that all guys remain engaged under every case.

Relate the model to tower and anchor selection

Ask how the selected analysis represents sag and tension changes and whether its assumptions hold for the governing operating and extreme conditions. Evaluate tower compression and anchor movement alongside lateral stiffness.

Do not remove sag by arbitrary tightening; that can increase loads elsewhere. The useful decision is whether the proposed guy system supplies the support the tower design assumes. No sag limit, pretension value or instruction to replace a cable with another material is provided by this FAQ or implied by MARSTON turbine rated power.